Change relpath() et al to return path by value

For AIO, and also some other recent patches, we need the ability to call
relpath() in a critical section. Until now that was not feasible, as it
allocated memory.

The fact that relpath() allocated memory also made it awkward to use in log
messages because we had to take care to free the memory afterwards. Which we
e.g. didn't do for when zeroing out an invalid buffer.

We discussed other solutions, e.g. filling a pre-allocated buffer that's
passed to relpath(), but they all came with plenty downsides or were larger
projects. The easiest fix seems to be to make relpath() return the path by
value.

To be able to return the path by value we need to determine the maximum length
of a relation path. This patch adds a long #define that computes the exact
maximum, which is verified to be correct in a regression test.

As this change the signature of relpath(), extensions using it will need to
adapt their code. We discussed leaving a backward-compat shim in place, but
decided it's not worth it given the use of relpath() doesn't seem widespread.

Discussion: https://postgr.es/m/xeri5mla4b5syjd5a25nok5iez2kr3bm26j2qn4u7okzof2bmf@kwdh2vf7npra

src/backend/access/rmgrdesc/smgrdesc.c

@@ -26,19 +26,17 @@ smgr_desc(StringInfo buf, XLogReaderState *record)
 	if (info == XLOG_SMGR_CREATE)
 	{
 		xl_smgr_create *xlrec = (xl_smgr_create *) rec;
-		char	   *path = relpathperm(xlrec->rlocator, xlrec->forkNum);
 
-		appendStringInfoString(buf, path);
+		appendStringInfoString(buf,
-		pfree(path);
+							   relpathperm(xlrec->rlocator, xlrec->forkNum).str);
 	}
 	else if (info == XLOG_SMGR_TRUNCATE)
 	{
 		xl_smgr_truncate *xlrec = (xl_smgr_truncate *) rec;
-		char	   *path = relpathperm(xlrec->rlocator, MAIN_FORKNUM);
 
-		appendStringInfo(buf, "%s to %u blocks flags %d", path,
+		appendStringInfo(buf, "%s to %u blocks flags %d",
+						 relpathperm(xlrec->rlocator, MAIN_FORKNUM).str,
 						 xlrec->blkno, xlrec->flags);
-		pfree(path);
 	}
 }
 

src/backend/access/rmgrdesc/xactdesc.c

@@ -287,10 +287,8 @@ xact_desc_relations(StringInfo buf, char *label, int nrels,
 		appendStringInfo(buf, "; %s:", label);
 		for (i = 0; i < nrels; i++)
 		{
-			char	   *path = relpathperm(xlocators[i], MAIN_FORKNUM);
+			appendStringInfo(buf, " %s",
-
+							 relpathperm(xlocators[i], MAIN_FORKNUM).str);
-			appendStringInfo(buf, " %s", path);
-			pfree(path);
 		}
 	}
 }

src/backend/access/transam/xlogutils.c

@@ -86,15 +86,14 @@ static void
 report_invalid_page(int elevel, RelFileLocator locator, ForkNumber forkno,
 					BlockNumber blkno, bool present)
 {
-	char	   *path = relpathperm(locator, forkno);
+	RelPathStr	path = relpathperm(locator, forkno);
 
 	if (present)
 		elog(elevel, "page %u of relation %s is uninitialized",
-			 blkno, path);
+			 blkno, path.str);
 	else
 		elog(elevel, "page %u of relation %s does not exist",
-			 blkno, path);
+			 blkno, path.str);
-	pfree(path);
 }
 
 /* Log a reference to an invalid page */
@@ -180,14 +179,9 @@ forget_invalid_pages(RelFileLocator locator, ForkNumber forkno,
 			hentry->key.forkno == forkno &&
 			hentry->key.blkno >= minblkno)
 		{
-			if (message_level_is_interesting(DEBUG2))
-			{
-				char	   *path = relpathperm(hentry->key.locator, forkno);
-
 			elog(DEBUG2, "page %u of relation %s has been dropped",
-					 hentry->key.blkno, path);
+				 hentry->key.blkno,
-				pfree(path);
+				 relpathperm(hentry->key.locator, forkno).str);
-			}
 
 			if (hash_search(invalid_page_tab,
 							&hentry->key,
@@ -213,14 +207,9 @@ forget_invalid_pages_db(Oid dbid)
 	{
 		if (hentry->key.locator.dbOid == dbid)
 		{
-			if (message_level_is_interesting(DEBUG2))
-			{
-				char	   *path = relpathperm(hentry->key.locator, hentry->key.forkno);
-
 			elog(DEBUG2, "page %u of relation %s has been dropped",
-					 hentry->key.blkno, path);
+				 hentry->key.blkno,
-				pfree(path);
+				 relpathperm(hentry->key.locator, hentry->key.forkno).str);
-			}
 
 			if (hash_search(invalid_page_tab,
 							&hentry->key,

src/backend/backup/basebackup_incremental.c

@@ -625,23 +625,21 @@ char *
 GetIncrementalFilePath(Oid dboid, Oid spcoid, RelFileNumber relfilenumber,
 					   ForkNumber forknum, unsigned segno)
 {
-	char	   *path;
+	RelPathStr	path;
 	char	   *lastslash;
 	char	   *ipath;
 
 	path = GetRelationPath(dboid, spcoid, relfilenumber, INVALID_PROC_NUMBER,
 						   forknum);
 
-	lastslash = strrchr(path, '/');
+	lastslash = strrchr(path.str, '/');
 	Assert(lastslash != NULL);
 	*lastslash = '\0';
 
 	if (segno > 0)
-		ipath = psprintf("%s/INCREMENTAL.%s.%u", path, lastslash + 1, segno);
+		ipath = psprintf("%s/INCREMENTAL.%s.%u", path.str, lastslash + 1, segno);
 	else
-		ipath = psprintf("%s/INCREMENTAL.%s", path, lastslash + 1);
+		ipath = psprintf("%s/INCREMENTAL.%s", path.str, lastslash + 1);
-
-	pfree(path);
 
 	return ipath;
 }

src/backend/catalog/catalog.c

@@ -528,7 +528,7 @@ RelFileNumber
 GetNewRelFileNumber(Oid reltablespace, Relation pg_class, char relpersistence)
 {
 	RelFileLocatorBackend rlocator;
-	char	   *rpath;
+	RelPathStr	rpath;
 	bool		collides;
 	ProcNumber	procNumber;
 
@@ -580,7 +580,7 @@ GetNewRelFileNumber(Oid reltablespace, Relation pg_class, char relpersistence)
 		/* Check for existing file of same name */
 		rpath = relpath(rlocator, MAIN_FORKNUM);
 
-		if (access(rpath, F_OK) == 0)
+		if (access(rpath.str, F_OK) == 0)
 		{
 			/* definite collision */
 			collides = true;
@@ -596,8 +596,6 @@ GetNewRelFileNumber(Oid reltablespace, Relation pg_class, char relpersistence)
 			 */
 			collides = false;
 		}
-
-		pfree(rpath);
 	} while (collides);
 
 	return rlocator.locator.relNumber;

src/backend/catalog/storage.c

@@ -524,14 +524,14 @@ RelationCopyStorage(SMgrRelation src, SMgrRelation dst,
 			 * (errcontext callbacks shouldn't be risking any such thing, but
 			 * people have been known to forget that rule.)
 			 */
-			char	   *relpath = relpathbackend(src->smgr_rlocator.locator,
+			RelPathStr	relpath = relpathbackend(src->smgr_rlocator.locator,
 												 src->smgr_rlocator.backend,
 												 forkNum);
 
 			ereport(ERROR,
 					(errcode(ERRCODE_DATA_CORRUPTED),
 					 errmsg("invalid page in block %u of relation %s",
-							blkno, relpath)));
+							blkno, relpath.str)));
 		}
 
 		/*

src/backend/replication/logical/reorderbuffer.c

@@ -2329,7 +2329,7 @@ ReorderBufferProcessTXN(ReorderBuffer *rb, ReorderBufferTXN *txn,
 					else if (reloid == InvalidOid)
 						elog(ERROR, "could not map filenumber \"%s\" to relation OID",
 							 relpathperm(change->data.tp.rlocator,
-										 MAIN_FORKNUM));
+										 MAIN_FORKNUM).str);
 
 					relation = RelationIdGetRelation(reloid);
 
@@ -2337,7 +2337,7 @@ ReorderBufferProcessTXN(ReorderBuffer *rb, ReorderBufferTXN *txn,
 						elog(ERROR, "could not open relation with OID %u (for filenumber \"%s\")",
 							 reloid,
 							 relpathperm(change->data.tp.rlocator,
-										 MAIN_FORKNUM));
+										 MAIN_FORKNUM).str);
 
 					if (!RelationIsLogicallyLogged(relation))
 						goto change_done;

src/backend/storage/buffer/bufmgr.c

@@ -1541,7 +1541,7 @@ WaitReadBuffers(ReadBuffersOperation *operation)
 							(errcode(ERRCODE_DATA_CORRUPTED),
 							 errmsg("invalid page in block %u of relation %s; zeroing out page",
 									io_first_block + j,
-									relpath(operation->smgr->smgr_rlocator, forknum))));
+									relpath(operation->smgr->smgr_rlocator, forknum).str)));
 					memset(bufBlock, 0, BLCKSZ);
 				}
 				else
@@ -1549,7 +1549,7 @@ WaitReadBuffers(ReadBuffersOperation *operation)
 							(errcode(ERRCODE_DATA_CORRUPTED),
 							 errmsg("invalid page in block %u of relation %s",
 									io_first_block + j,
-									relpath(operation->smgr->smgr_rlocator, forknum))));
+									relpath(operation->smgr->smgr_rlocator, forknum).str)));
 			}
 
 			/* Terminate I/O and set BM_VALID. */
@@ -2284,7 +2284,7 @@ ExtendBufferedRelShared(BufferManagerRelation bmr,
 		ereport(ERROR,
 				(errcode(ERRCODE_PROGRAM_LIMIT_EXCEEDED),
 				 errmsg("cannot extend relation %s beyond %u blocks",
-						relpath(bmr.smgr->smgr_rlocator, fork),
+						relpath(bmr.smgr->smgr_rlocator, fork).str,
 						MaxBlockNumber)));
 
 	/*
@@ -2355,7 +2355,8 @@ ExtendBufferedRelShared(BufferManagerRelation bmr,
 			if (valid && !PageIsNew((Page) buf_block))
 				ereport(ERROR,
 						(errmsg("unexpected data beyond EOF in block %u of relation %s",
-								existing_hdr->tag.blockNum, relpath(bmr.smgr->smgr_rlocator, fork)),
+								existing_hdr->tag.blockNum,
+								relpath(bmr.smgr->smgr_rlocator, fork).str),
 						 errhint("This has been seen to occur with buggy kernels; consider updating your system.")));
 
 			/*
@@ -3663,7 +3664,6 @@ DebugPrintBufferRefcount(Buffer buffer)
 {
 	BufferDesc *buf;
 	int32		loccount;
-	char	   *path;
 	char	   *result;
 	ProcNumber	backend;
 	uint32		buf_state;
@@ -3683,15 +3683,14 @@ DebugPrintBufferRefcount(Buffer buffer)
 	}
 
 	/* theoretically we should lock the bufhdr here */
-	path = relpathbackend(BufTagGetRelFileLocator(&buf->tag), backend,
-						  BufTagGetForkNum(&buf->tag));
 	buf_state = pg_atomic_read_u32(&buf->state);
 
 	result = psprintf("[%03d] (rel=%s, blockNum=%u, flags=0x%x, refcount=%u %d)",
-					  buffer, path,
+					  buffer,
+					  relpathbackend(BufTagGetRelFileLocator(&buf->tag), backend,
+									 BufTagGetForkNum(&buf->tag)).str,
 					  buf->tag.blockNum, buf_state & BUF_FLAG_MASK,
 					  BUF_STATE_GET_REFCOUNT(buf_state), loccount);
-	pfree(path);
 	return result;
 }
 
@@ -5611,16 +5610,13 @@ AbortBufferIO(Buffer buffer)
 		if (buf_state & BM_IO_ERROR)
 		{
 			/* Buffer is pinned, so we can read tag without spinlock */
-			char	   *path;
-
-			path = relpathperm(BufTagGetRelFileLocator(&buf_hdr->tag),
-							   BufTagGetForkNum(&buf_hdr->tag));
 			ereport(WARNING,
 					(errcode(ERRCODE_IO_ERROR),
 					 errmsg("could not write block %u of %s",
-							buf_hdr->tag.blockNum, path),
+							buf_hdr->tag.blockNum,
+							relpathperm(BufTagGetRelFileLocator(&buf_hdr->tag),
+										BufTagGetForkNum(&buf_hdr->tag)).str),
 					 errdetail("Multiple failures --- write error might be permanent.")));
-			pfree(path);
 		}
 	}
 
@@ -5637,14 +5633,10 @@ shared_buffer_write_error_callback(void *arg)
 
 	/* Buffer is pinned, so we can read the tag without locking the spinlock */
 	if (bufHdr != NULL)
-	{
-		char	   *path = relpathperm(BufTagGetRelFileLocator(&bufHdr->tag),
-									   BufTagGetForkNum(&bufHdr->tag));
-
 		errcontext("writing block %u of relation %s",
-				   bufHdr->tag.blockNum, path);
+				   bufHdr->tag.blockNum,
-		pfree(path);
+				   relpathperm(BufTagGetRelFileLocator(&bufHdr->tag),
-	}
+							   BufTagGetForkNum(&bufHdr->tag)).str);
 }
 
 /*
@@ -5656,15 +5648,11 @@ local_buffer_write_error_callback(void *arg)
 	BufferDesc *bufHdr = (BufferDesc *) arg;
 
 	if (bufHdr != NULL)
-	{
-		char	   *path = relpathbackend(BufTagGetRelFileLocator(&bufHdr->tag),
-										  MyProcNumber,
-										  BufTagGetForkNum(&bufHdr->tag));
-
 		errcontext("writing block %u of relation %s",
-				   bufHdr->tag.blockNum, path);
+				   bufHdr->tag.blockNum,
-		pfree(path);
+				   relpathbackend(BufTagGetRelFileLocator(&bufHdr->tag),
-	}
+								  MyProcNumber,
+								  BufTagGetForkNum(&bufHdr->tag)).str);
 }
 
 /*

src/backend/storage/buffer/localbuf.c

@@ -360,7 +360,7 @@ ExtendBufferedRelLocal(BufferManagerRelation bmr,
 		ereport(ERROR,
 				(errcode(ERRCODE_PROGRAM_LIMIT_EXCEEDED),
 				 errmsg("cannot extend relation %s beyond %u blocks",
-						relpath(bmr.smgr->smgr_rlocator, fork),
+						relpath(bmr.smgr->smgr_rlocator, fork).str,
 						MaxBlockNumber)));
 
 	for (uint32 i = 0; i < extend_by; i++)
@@ -510,7 +510,7 @@ DropRelationLocalBuffers(RelFileLocator rlocator, ForkNumber forkNum,
 					 bufHdr->tag.blockNum,
 					 relpathbackend(BufTagGetRelFileLocator(&bufHdr->tag),
 									MyProcNumber,
-									BufTagGetForkNum(&bufHdr->tag)),
+									BufTagGetForkNum(&bufHdr->tag)).str,
 					 LocalRefCount[i]);
 
 			/* Remove entry from hashtable */
@@ -555,7 +555,7 @@ DropRelationAllLocalBuffers(RelFileLocator rlocator)
 					 bufHdr->tag.blockNum,
 					 relpathbackend(BufTagGetRelFileLocator(&bufHdr->tag),
 									MyProcNumber,
-									BufTagGetForkNum(&bufHdr->tag)),
+									BufTagGetForkNum(&bufHdr->tag)).str,
 					 LocalRefCount[i]);
 			/* Remove entry from hashtable */
 			hresult = (LocalBufferLookupEnt *)

src/backend/storage/smgr/md.c

@@ -182,7 +182,7 @@ void
 mdcreate(SMgrRelation reln, ForkNumber forknum, bool isRedo)
 {
 	MdfdVec    *mdfd;
-	char	   *path;
+	RelPathStr	path;
 	File		fd;
 
 	if (isRedo && reln->md_num_open_segs[forknum] > 0)
@@ -205,26 +205,24 @@ mdcreate(SMgrRelation reln, ForkNumber forknum, bool isRedo)
 
 	path = relpath(reln->smgr_rlocator, forknum);
 
-	fd = PathNameOpenFile(path, _mdfd_open_flags() | O_CREAT | O_EXCL);
+	fd = PathNameOpenFile(path.str, _mdfd_open_flags() | O_CREAT | O_EXCL);
 
 	if (fd < 0)
 	{
 		int			save_errno = errno;
 
 		if (isRedo)
-			fd = PathNameOpenFile(path, _mdfd_open_flags());
+			fd = PathNameOpenFile(path.str, _mdfd_open_flags());
 		if (fd < 0)
 		{
 			/* be sure to report the error reported by create, not open */
 			errno = save_errno;
 			ereport(ERROR,
 					(errcode_for_file_access(),
-					 errmsg("could not create file \"%s\": %m", path)));
+					 errmsg("could not create file \"%s\": %m", path.str)));
 		}
 	}
 
-	pfree(path);
-
 	_fdvec_resize(reln, forknum, 1);
 	mdfd = &reln->md_seg_fds[forknum][0];
 	mdfd->mdfd_vfd = fd;
@@ -335,7 +333,7 @@ do_truncate(const char *path)
 static void
 mdunlinkfork(RelFileLocatorBackend rlocator, ForkNumber forknum, bool isRedo)
 {
-	char	   *path;
+	RelPathStr	path;
 	int			ret;
 	int			save_errno;
 
@@ -351,7 +349,7 @@ mdunlinkfork(RelFileLocatorBackend rlocator, ForkNumber forknum, bool isRedo)
 		if (!RelFileLocatorBackendIsTemp(rlocator))
 		{
 			/* Prevent other backends' fds from holding on to the disk space */
-			ret = do_truncate(path);
+			ret = do_truncate(path.str);
 
 			/* Forget any pending sync requests for the first segment */
 			save_errno = errno;
@@ -364,13 +362,13 @@ mdunlinkfork(RelFileLocatorBackend rlocator, ForkNumber forknum, bool isRedo)
 		/* Next unlink the file, unless it was already found to be missing */
 		if (ret >= 0 || errno != ENOENT)
 		{
-			ret = unlink(path);
+			ret = unlink(path.str);
 			if (ret < 0 && errno != ENOENT)
 			{
 				save_errno = errno;
 				ereport(WARNING,
 						(errcode_for_file_access(),
-						 errmsg("could not remove file \"%s\": %m", path)));
+						 errmsg("could not remove file \"%s\": %m", path.str)));
 				errno = save_errno;
 			}
 		}
@@ -378,7 +376,7 @@ mdunlinkfork(RelFileLocatorBackend rlocator, ForkNumber forknum, bool isRedo)
 	else
 	{
 		/* Prevent other backends' fds from holding on to the disk space */
-		ret = do_truncate(path);
+		ret = do_truncate(path.str);
 
 		/* Register request to unlink first segment later */
 		save_errno = errno;
@@ -400,12 +398,12 @@ mdunlinkfork(RelFileLocatorBackend rlocator, ForkNumber forknum, bool isRedo)
 	 */
 	if (ret >= 0 || errno != ENOENT)
 	{
-		char	   *segpath = (char *) palloc(strlen(path) + 12);
+		char	   *segpath = (char *) palloc(strlen(path.str) + 12);
 		BlockNumber segno;
 
 		for (segno = 1;; segno++)
 		{
-			sprintf(segpath, "%s.%u", path, segno);
+			sprintf(segpath, "%s.%u", path.str, segno);
 
 			if (!RelFileLocatorBackendIsTemp(rlocator))
 			{
@@ -435,8 +433,6 @@ mdunlinkfork(RelFileLocatorBackend rlocator, ForkNumber forknum, bool isRedo)
 		}
 		pfree(segpath);
 	}
-
-	pfree(path);
 }
 
 /*
@@ -475,7 +471,7 @@ mdextend(SMgrRelation reln, ForkNumber forknum, BlockNumber blocknum,
 		ereport(ERROR,
 				(errcode(ERRCODE_PROGRAM_LIMIT_EXCEEDED),
 				 errmsg("cannot extend file \"%s\" beyond %u blocks",
-						relpath(reln->smgr_rlocator, forknum),
+						relpath(reln->smgr_rlocator, forknum).str,
 						InvalidBlockNumber)));
 
 	v = _mdfd_getseg(reln, forknum, blocknum, skipFsync, EXTENSION_CREATE);
@@ -537,7 +533,7 @@ mdzeroextend(SMgrRelation reln, ForkNumber forknum,
 		ereport(ERROR,
 				(errcode(ERRCODE_PROGRAM_LIMIT_EXCEEDED),
 				 errmsg("cannot extend file \"%s\" beyond %u blocks",
-						relpath(reln->smgr_rlocator, forknum),
+						relpath(reln->smgr_rlocator, forknum).str,
 						InvalidBlockNumber)));
 
 	while (remblocks > 0)
@@ -629,7 +625,7 @@ static MdfdVec *
 mdopenfork(SMgrRelation reln, ForkNumber forknum, int behavior)
 {
 	MdfdVec    *mdfd;
-	char	   *path;
+	RelPathStr	path;
 	File		fd;
 
 	/* No work if already open */
@@ -638,23 +634,18 @@ mdopenfork(SMgrRelation reln, ForkNumber forknum, int behavior)
 
 	path = relpath(reln->smgr_rlocator, forknum);
 
-	fd = PathNameOpenFile(path, _mdfd_open_flags());
+	fd = PathNameOpenFile(path.str, _mdfd_open_flags());
 
 	if (fd < 0)
 	{
 		if ((behavior & EXTENSION_RETURN_NULL) &&
 			FILE_POSSIBLY_DELETED(errno))
-		{
-			pfree(path);
 			return NULL;
-		}
 		ereport(ERROR,
 				(errcode_for_file_access(),
-				 errmsg("could not open file \"%s\": %m", path)));
+				 errmsg("could not open file \"%s\": %m", path.str)));
 	}
 
-	pfree(path);
-
 	_fdvec_resize(reln, forknum, 1);
 	mdfd = &reln->md_seg_fds[forknum][0];
 	mdfd->mdfd_vfd = fd;
@@ -1176,7 +1167,7 @@ mdtruncate(SMgrRelation reln, ForkNumber forknum,
 			return;
 		ereport(ERROR,
 				(errmsg("could not truncate file \"%s\" to %u blocks: it's only %u blocks now",
-						relpath(reln->smgr_rlocator, forknum),
+						relpath(reln->smgr_rlocator, forknum).str,
 						nblocks, curnblk)));
 	}
 	if (nblocks == curnblk)
@@ -1540,18 +1531,15 @@ _fdvec_resize(SMgrRelation reln,
 static char *
 _mdfd_segpath(SMgrRelation reln, ForkNumber forknum, BlockNumber segno)
 {
-	char	   *path,
+	RelPathStr	path;
-			   *fullpath;
+	char	   *fullpath;
 
 	path = relpath(reln->smgr_rlocator, forknum);
 
 	if (segno > 0)
-	{
+		fullpath = psprintf("%s.%u", path.str, segno);
-		fullpath = psprintf("%s.%u", path, segno);
-		pfree(path);
-	}
 	else
-		fullpath = path;
+		fullpath = pstrdup(path.str);
 
 	return fullpath;
 }
@@ -1811,12 +1799,11 @@ mdsyncfiletag(const FileTag *ftag, char *path)
 int
 mdunlinkfiletag(const FileTag *ftag, char *path)
 {
-	char	   *p;
+	RelPathStr	p;
 
 	/* Compute the path. */
 	p = relpathperm(ftag->rlocator, MAIN_FORKNUM);
-	strlcpy(path, p, MAXPGPATH);
+	strlcpy(path, p.str, MAXPGPATH);
-	pfree(p);
 
 	/* Try to unlink the file. */
 	return unlink(path);

src/backend/utils/adt/dbsize.c

@@ -326,7 +326,7 @@ static int64
 calculate_relation_size(RelFileLocator *rfn, ProcNumber backend, ForkNumber forknum)
 {
 	int64		totalsize = 0;
-	char	   *relationpath;
+	RelPathStr	relationpath;
 	char		pathname[MAXPGPATH];
 	unsigned int segcount = 0;
 
@@ -340,10 +340,10 @@ calculate_relation_size(RelFileLocator *rfn, ProcNumber backend, ForkNumber fork
 
 		if (segcount == 0)
 			snprintf(pathname, MAXPGPATH, "%s",
-					 relationpath);
+					 relationpath.str);
 		else
 			snprintf(pathname, MAXPGPATH, "%s.%u",
-					 relationpath, segcount);
+					 relationpath.str, segcount);
 
 		if (stat(pathname, &fst) < 0)
 		{
@@ -973,7 +973,7 @@ pg_relation_filepath(PG_FUNCTION_ARGS)
 	Form_pg_class relform;
 	RelFileLocator rlocator;
 	ProcNumber	backend;
-	char	   *path;
+	RelPathStr	path;
 
 	tuple = SearchSysCache1(RELOID, ObjectIdGetDatum(relid));
 	if (!HeapTupleIsValid(tuple))
@@ -1039,5 +1039,5 @@ pg_relation_filepath(PG_FUNCTION_ARGS)
 
 	path = relpathbackend(rlocator, backend, MAIN_FORKNUM);
 
-	PG_RETURN_TEXT_P(cstring_to_text(path));
+	PG_RETURN_TEXT_P(cstring_to_text(path.str));
 }

src/bin/pg_rewind/filemap.c

@@ -652,18 +652,17 @@ isRelDataFile(const char *path)
 static char *
 datasegpath(RelFileLocator rlocator, ForkNumber forknum, BlockNumber segno)
 {
-	char	   *path;
+	RelPathStr	path;
 	char	   *segpath;
 
 	path = relpathperm(rlocator, forknum);
 	if (segno > 0)
 	{
-		segpath = psprintf("%s.%u", path, segno);
+		segpath = psprintf("%s.%u", path.str, segno);
-		pfree(path);
 		return segpath;
 	}
 	else
-		return path;
+		return pstrdup(path.str);
 }
 
 /*

src/common/relpath.c

@@ -132,17 +132,18 @@ GetDatabasePath(Oid dbOid, Oid spcOid)
 /*
  * GetRelationPath - construct path to a relation's file
  *
- * Result is a palloc'd string.
+ * The result is returned in-place as a struct, to make it suitable for use in
+ * critical sections etc.
  *
  * Note: ideally, procNumber would be declared as type ProcNumber, but
  * relpath.h would have to include a backend-only header to do that; doesn't
  * seem worth the trouble considering ProcNumber is just int anyway.
  */
-char *
+RelPathStr
 GetRelationPath(Oid dbOid, Oid spcOid, RelFileNumber relNumber,
 				int procNumber, ForkNumber forkNumber)
 {
-	char	   *path;
+	RelPathStr	rp;
 
 	if (spcOid == GLOBALTABLESPACE_OID)
 	{
@@ -150,10 +151,11 @@ GetRelationPath(Oid dbOid, Oid spcOid, RelFileNumber relNumber,
 		Assert(dbOid == 0);
 		Assert(procNumber == INVALID_PROC_NUMBER);
 		if (forkNumber != MAIN_FORKNUM)
-			path = psprintf("global/%u_%s",
+			sprintf(rp.str, "global/%u_%s",
 					relNumber, forkNames[forkNumber]);
 		else
-			path = psprintf("global/%u", relNumber);
+			sprintf(rp.str, "global/%u",
+					relNumber);
 	}
 	else if (spcOid == DEFAULTTABLESPACE_OID)
 	{
@@ -161,21 +163,23 @@ GetRelationPath(Oid dbOid, Oid spcOid, RelFileNumber relNumber,
 		if (procNumber == INVALID_PROC_NUMBER)
 		{
 			if (forkNumber != MAIN_FORKNUM)
-				path = psprintf("base/%u/%u_%s",
+			{
+				sprintf(rp.str, "base/%u/%u_%s",
 						dbOid, relNumber,
 						forkNames[forkNumber]);
+			}
 			else
-				path = psprintf("base/%u/%u",
+				sprintf(rp.str, "base/%u/%u",
 						dbOid, relNumber);
 		}
 		else
 		{
 			if (forkNumber != MAIN_FORKNUM)
-				path = psprintf("base/%u/t%d_%u_%s",
+				sprintf(rp.str, "base/%u/t%d_%u_%s",
 						dbOid, procNumber, relNumber,
 						forkNames[forkNumber]);
 			else
-				path = psprintf("base/%u/t%d_%u",
+				sprintf(rp.str, "base/%u/t%d_%u",
 						dbOid, procNumber, relNumber);
 		}
 	}
@@ -185,13 +189,13 @@ GetRelationPath(Oid dbOid, Oid spcOid, RelFileNumber relNumber,
 		if (procNumber == INVALID_PROC_NUMBER)
 		{
 			if (forkNumber != MAIN_FORKNUM)
-				path = psprintf("%s/%u/%s/%u/%u_%s",
+				sprintf(rp.str, "%s/%u/%s/%u/%u_%s",
 						PG_TBLSPC_DIR, spcOid,
 						TABLESPACE_VERSION_DIRECTORY,
 						dbOid, relNumber,
 						forkNames[forkNumber]);
 			else
-				path = psprintf("%s/%u/%s/%u/%u",
+				sprintf(rp.str, "%s/%u/%s/%u/%u",
 						PG_TBLSPC_DIR, spcOid,
 						TABLESPACE_VERSION_DIRECTORY,
 						dbOid, relNumber);
@@ -199,17 +203,20 @@ GetRelationPath(Oid dbOid, Oid spcOid, RelFileNumber relNumber,
 		else
 		{
 			if (forkNumber != MAIN_FORKNUM)
-				path = psprintf("%s/%u/%s/%u/t%d_%u_%s",
+				sprintf(rp.str, "%s/%u/%s/%u/t%d_%u_%s",
 						PG_TBLSPC_DIR, spcOid,
 						TABLESPACE_VERSION_DIRECTORY,
 						dbOid, procNumber, relNumber,
 						forkNames[forkNumber]);
 			else
-				path = psprintf("%s/%u/%s/%u/t%d_%u",
+				sprintf(rp.str, "%s/%u/%s/%u/t%d_%u",
 						PG_TBLSPC_DIR, spcOid,
 						TABLESPACE_VERSION_DIRECTORY,
 						dbOid, procNumber, relNumber);
 		}
 	}
-	return path;
+
+	Assert(strnlen(rp.str, REL_PATH_STR_MAXLEN + 1) <= REL_PATH_STR_MAXLEN);
+
+	return rp;
 }

src/include/common/relpath.h

@@ -77,12 +77,60 @@ extern PGDLLIMPORT const char *const forkNames[];
 extern ForkNumber forkname_to_number(const char *forkName);
 extern int	forkname_chars(const char *str, ForkNumber *fork);
 
+
+/*
+ * Unfortunately, there's no easy way to derive PROCNUMBER_CHARS from
+ * MAX_BACKENDS. MAX_BACKENDS is 2^18-1. Crosschecked in test_relpath().
+ */
+#define PROCNUMBER_CHARS	6
+
+/*
+ * The longest possible relation path lengths is from the following format:
+ * sprintf(rp.path, "%s/%u/%s/%u/t%d_%u",
+ *         PG_TBLSPC_DIR, spcOid,
+ *         TABLESPACE_VERSION_DIRECTORY,
+ *         dbOid, procNumber, relNumber);
+ *
+ * Note this does *not* include the trailing null-byte, to make it easier to
+ * combine it with other lengths.
+ */
+#define REL_PATH_STR_MAXLEN \
+	( \
+		sizeof(PG_TBLSPC_DIR) - 1 \
+		+ sizeof((char)'/') \
+		+ OIDCHARS /* spcOid */ \
+		+ sizeof((char)'/') \
+		+ sizeof(TABLESPACE_VERSION_DIRECTORY) - 1 \
+		+ sizeof((char)'/') \
+		+ OIDCHARS /* dbOid */ \
+		+ sizeof((char)'/') \
+		+ sizeof((char)'t') /* temporary table indicator */ \
+		+ PROCNUMBER_CHARS /* procNumber */ \
+		+ sizeof((char)'_') \
+		+ OIDCHARS /* relNumber */ \
+		+ sizeof((char)'_') \
+		+ FORKNAMECHARS /* forkNames[forkNumber] */ \
+	)
+
+/*
+ * String of the exact length required to represent a relation path. We return
+ * this struct, instead of char[REL_PATH_STR_MAXLEN + 1], as the pointer would
+ * decay to a plain char * too easily, possibly preventing the compiler from
+ * detecting invalid references to the on-stack return value of
+ * GetRelationPath().
+ */
+typedef struct RelPathStr
+{
+	char		str[REL_PATH_STR_MAXLEN + 1];
+} RelPathStr;
+
+
 /*
  * Stuff for computing filesystem pathnames for relations.
  */
 extern char *GetDatabasePath(Oid dbOid, Oid spcOid);
 
-extern char *GetRelationPath(Oid dbOid, Oid spcOid, RelFileNumber relNumber,
+extern RelPathStr GetRelationPath(Oid dbOid, Oid spcOid, RelFileNumber relNumber,
 								  int procNumber, ForkNumber forkNumber);
 
 /*

src/test/regress/expected/misc_functions.out

@@ -903,3 +903,14 @@ SELECT gist_stratnum_common(3);
                    18
 (1 row)
 
+-- relpath tests
+CREATE FUNCTION test_relpath()
+    RETURNS void
+    AS :'regresslib'
+    LANGUAGE C;
+SELECT test_relpath();
+ test_relpath 
+--------------
+ 
+(1 row)
+

src/test/regress/regress.c

@@ -36,6 +36,7 @@
 #include "optimizer/plancat.h"
 #include "parser/parse_coerce.h"
 #include "port/atomics.h"
+#include "postmaster/postmaster.h"	/* for MAX_BACKENDS */
 #include "storage/spin.h"
 #include "utils/array.h"
 #include "utils/builtins.h"
@@ -1208,3 +1209,31 @@ binary_coercible(PG_FUNCTION_ARGS)
 
 	PG_RETURN_BOOL(IsBinaryCoercible(srctype, targettype));
 }
+
+/*
+ * Sanity checks for functions in relpath.h
+ */
+PG_FUNCTION_INFO_V1(test_relpath);
+Datum
+test_relpath(PG_FUNCTION_ARGS)
+{
+	RelPathStr	rpath;
+
+	/*
+	 * Verify that PROCNUMBER_CHARS and MAX_BACKENDS stay in sync.
+	 * Unfortunately I don't know how to express that in a way suitable for a
+	 * static assert.
+	 */
+	if ((int) ceil(log10(MAX_BACKENDS)) != PROCNUMBER_CHARS)
+		elog(WARNING, "mismatch between MAX_BACKENDS and PROCNUMBER_CHARS");
+
+	/* verify that the max-length relpath is generated ok */
+	rpath = GetRelationPath(OID_MAX, OID_MAX, OID_MAX, MAX_BACKENDS - 1,
+							INIT_FORKNUM);
+
+	if (strlen(rpath.str) != REL_PATH_STR_MAXLEN)
+		elog(WARNING, "maximum length relpath is if length %zu instead of %zu",
+			 strlen(rpath.str), REL_PATH_STR_MAXLEN);
+
+	PG_RETURN_VOID();
+}

src/test/regress/sql/misc_functions.sql

@@ -403,3 +403,11 @@ DROP FUNCTION explain_mask_costs(text, bool, bool, bool, bool);
 -- test stratnum support functions
 SELECT gist_stratnum_common(7);
 SELECT gist_stratnum_common(3);
+
+
+-- relpath tests
+CREATE FUNCTION test_relpath()
+    RETURNS void
+    AS :'regresslib'
+    LANGUAGE C;
+SELECT test_relpath();

src/tools/pgindent/typedefs.list

@@ -2410,6 +2410,7 @@ RelMapFile
 RelMapping
 RelOptInfo
 RelOptKind
+RelPathStr
 RelStatsInfo
 RelToCheck
 RelToCluster